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Help with porting a cab?

Discussion in 'Amps and Cabs [BG]' started by John Bigboote, Mar 16, 2009.

  1. A while back, I got a very good deal on an Ampeg SVT-48HE, or so it seemed. The previous owner had replaced the scrawny stock 50W speakers with Beta-8As, making the cab good for a whopping 900W at 8 ohms. But for one reason or another, I'd not had many chances to gig with it until lately. And at high volumes, as you might expect, it occasionally farts like a trucker.

    Now, I love the concept of a 4x8 cab; I love the idea of running it with a 115 to cover the mids and upper-mids with punch and detail. And my current rig is biamped through a PLX-1804, so I could send only signals higher than, say, 200Hz to the 48, which I guess should alleviate the problem, but I figure the *right* thing to do is run the numbers (WinISD?) and determine how the cab should be ported with those new drivers. But, uh, I haven't figured out how to do that.

    The internal dimensions of the cab are 46.5 cm H x 56.5 cm W x 30.0 cm D. There's a single 4x4x30 cm piece of wood bracing the front panel from the back panel. According to my bad math, that gives me an internal volume of 78,337.5 cc (guess that's 78.34 l). Oh, and there's 3 cm-thick batting all around the inside; not sure whether to subtract that or not.

    The driver specs are here:

    There are two ports, both 7 cm internal diameter, by 8 cm long. I can't imagine that's enough porting for these four drivers.

    Can anyone tell me whether I need to cut more port into that cab to help alleviate the farting problem? Or should I just resign myself to the fact that this cab, no matter what, can't drive low frequencies at high volumes, and just use low-cut to keep it up in the mids?


  2. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    crap driver for the intended purpose.
  3. great...
  4. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Turn it into a sealed midrange only cab and you'll fare better with those drivers. You probably have too much internal volume to get the best response for your crossover (I haven't checked) but that can be solved by checking out net volume in WinISD and some styrofoam and glue to fill space before lightly stuffing with polyfiber batting.
  5. At the risk of offending you or anyone else with my ignorance, can you briefly explain why those drivers aren't suitable for a ported bass cabinet? Assuming I input the numbers into WinISD correctly (and no, that's not a safe assumption), I get a cab volume of 76.5 l, compared to the actual 78.34 l. But the port numbers seem screwy, hence my puzzlement. What is it about these drivers that makes them, in your estimation, inappropriate for this application? BTW, WinISD also seems to suggest that, for a sealed model, the volume should be 37.9 l, almost exactly half of what I've got now...

    Thanks for helping me through this,

  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Insufficient xmax. Run it through WinISD Alpha Pro and look at the Maximum Power and Maximum SPL charts. The SPL chart in and of itself tells you only small signal response, not output capability. The 225w driver power rating is of little consequence when it's displacement limited to less than 50 watts from 70 to 150Hz.

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